1. Field of the Invention
The present invention relates to a method and apparatus for converting solar energy to electric energy, and in particular, to a method and apparatus which focus solar energy onto an absorbing receiver body to cause the body to emit infrared energy, and which tailor the spectrum of the infrared energy for application to photovoltaic (PV) cells for high-efficiency conversion to electric energy as generally illustrated at the top of FIG. 3 of the drawings. The invention is usefull in both terrestrial and space applications, and provides for storing energy for conversion to electrical power when needed. It also provides for use of an alternative heat source when solar energy is not available.
2. Description of the Related Art
FIG. 1 illustrates the measured peak power efficiency of the kinematic Stirling dish, which has previously set the standard for the highest solar power conversion efficiency. As shown, a unit available irradiance A is designated 1.0; the component dish concentrator reflectivity efficiency B is 0.92; the component intercept efficiency C is 0.97; the component receiver efficiency D is 0.90; the component power conversion efficiency E is 0.39; and the component parasitic efficiency F is 0.96. This results in a cumulative efficiency of 30.4%. That is, about 30% of the incident power is applied, for example, to a space vehicle or utility grid line. This is much higher than other conventional concentrating or non-concentrating photovoltaic (PV) systems, the Bryton system, the Rankin system or the Stirling free-piston system (typically less than 22% efficiency). The Stirling engine does, however, suffer drawbacks. For example, the efficiency of the Stirling engine, having heat transferring gas flowing therethrough, is sensitive to solar flux level distribution. Further, in the tube impingement receiver of the Stirling engine, each facet must be aimed at different points on the receiver in order to obtain high efficiency and long receiver life. This increases manufacturing costs. In addition, the overall mass of the Stirling engine is relatively high.
In fact, numerous techniques are known for conversion of solar energy to electric energy, all of which suffer drawbacks ranging from low conversion efficiencies to high manufacturing and/or maintenance costs. For example, solar power dynamic systems have moving parts which tend to reduce reliability and increase maintenance costs. Also, conventional photovoltaic systems (solar cells) and most dynamic systems lack the capability to produce electricity during periods of cloud cover or after the sun has set. Moreover, the direct impingement receiver, the heat pipe receiver, and the pool boil receiver require relatively expensive and toxic materials and costly manufacturing processes.